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9990 · 1.1.1

Diagnostic criteria for schizophrenia — practice questions

Practice and worked examples for 9990 Diagnostic criteria for schizophrenia. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A 22-year-old reports hearing voices commenting on their behaviour for six weeks. They believe neighbours are controlling their thoughts. Affect is flat and they have stopped attending university.

(a) Outline the positive and negative symptoms shown. [4 marks] (b) Using Rosenhan (1973), evaluate the validity of schizophrenia diagnosis. [6 marks]

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(a) AO1 — Positive symptoms:

  • Auditory hallucinations — hearing voices commenting on behaviour.
  • Delusions — belief that neighbours control thoughts (delusion of control/persecution).

AO1 — Negative symptoms:

  • Flat affect — reduced emotional expression.
  • Avolition — withdrawal from university, loss of goal-directed activity.

(b) AO1 — Rosenhan (1973): Eight sane pseudopatients presented vague symptoms and were all admitted with schizophrenia diagnoses; once inside, normal behaviour was reinterpreted within a psychiatric label.

AO3 — Evaluation:

  • Supports invalidity: Shows diagnosis may be over-inclusive and context-dependent — staff expected illness and interpreted normal behaviour pathologically.
  • Methodological critique: Small sample, dated institutions, deception ethics — limits generalisability to modern DSM-5/ICD-11 practice.
  • Counter-evidence: Improved structured interviews (e.g. SCAN) and explicit criteria aim to increase inter-rater reliability — Rosenhan highlights risk, not inevitability, of misdiagnosis.

Judgement: Rosenhan usefully warns that validity depends on distinguishing genuine psychosis from situational mislabelling — but modern criteria and duration rules (as in this case) aim to reduce false positives.

Worked example 2

Two clinicians, Dr. Smith and Dr. Jones, independently diagnose 100 patients referred for suspected psychosis. They use DSM-5 criteria to decide whether each patient has schizophrenia. The results are shown in the table below.

Dr. Jones: SchizophreniaDr. Jones: Not SchizophreniaTotal
Dr. Smith: Schizophrenia351045
------------
Dr. Smith: Not Schizophrenia55055
Total4060100

Calculate the inter-rater reliability using Cohen's Kappa (κ) and briefly interpret the result. [8 marks]

Show solution outline

Step 1: Calculate Observed Agreement (Po) This is the proportion of cases where the clinicians agreed.

  • Agreed on 'Schizophrenia': 35 patients
  • Agreed on 'Not Schizophrenia': 50 patients
  • Total agreement = 35 + 50 = 85 patients
  • Po = Total Agreement / Total Patients = 85 / 100 = 0.85

Step 2: Calculate Chance Agreement (Pe) This is the probability they would agree by chance alone.

  • Probability Dr. Smith says 'Schizophrenia' = 45 / 100 = 0.45

  • Probability Dr. Jones says 'Schizophrenia' = 40 / 100 = 0.40

  • Chance agreement for 'Schizophrenia' = 0.45 * 0.40 = 0.18

  • Probability Dr. Smith says 'Not Schizophrenia' = 55 / 100 = 0.55

  • Probability Dr. Jones says 'Not Schizophrenia' = 60 / 100 = 0.60

  • Chance agreement for 'Not Schizophrenia' = 0.55 * 0.60 = 0.33

  • Total chance agreement (Pe) = 0.18 + 0.33 = 0.51

Step 3: Calculate Cohen's Kappa (κ) The formula is κ = (Po - Pe) / (1 - Pe).

  • κ = (0.85 - 0.51) / (1 - 0.51)
  • κ = 0.34 / 0.49
  • κ ≈ 0.694

Step 4: Interpretation

  • The calculated Cohen's Kappa is κ = 0.69 (to 2 d.p.).
  • According to standard benchmarks (e.g., Landis & Koch), a kappa value between 0.61 and 0.80 indicates 'substantial' agreement.
  • This suggests that for this sample of patients, the DSM-5 criteria for schizophrenia have good inter-rater reliability, as the level of agreement between the two clinicians was well above what would be expected by chance.